Access the XRef data correctly in the "GetStats" handler in src/core/worker.js

Even though the code obviously works as-is, given that we have unit-tests for it, it still feels incorrect to just *assume* that the `XRef`-instance has all of its properties immediately available. Especially when (almost) all of the other handlers, in `src/core/worker.js`, protect their data accesses with appropriate `pdfManager.ensure` calls.

src/core/worker.js

@@ -507,7 +507,7 @@ class WorkerMessageHandler {
     });
 
     handler.on("GetStats", function wphSetupGetStats(data) {
-      return pdfManager.pdfDocument.xref.stats;
+      return pdfManager.ensureXRef("stats");
     });
 
     handler.on("GetAnnotations", function ({ pageIndex, intent }) {